The present invention relates to a surge protection circuit for a switching mode power supply, and more particularly, to a surge protection circuit for a power supply in a switching mode capable of snubbing primary and secondary currents and surge voltages at both the primary and the secondary windings of a switching power supply transformer.
Switching mode power supplies are generally well known in the art to include a switching transistor driven in response to a drive signal, for generating a switching pulse signal in order to control a power supply transformer in an effort to output a voltage having a constant or fixed amplitude. Generally, these switching mode power supplies require a protection circuit to protect the load, or to protect components within the power supply from an overvoltage, an overcurrent or an induced surge. Such switching mode power supplies are discussed in U.S. Pat. No. 5,227,964 to Shoichi Furuhata entitled Switching Power Supply With Overcurrent Protection Circuit. Furuhata's '964 patent illustrates switching mode power supply having a resonant capacitor and a surge absorber connected in parallel with the primary winding of the power supply transformer. The surge absorber used to protect the switching transistor. An overcurrent protective circuit is connected to the power supply to detect overcurrent on the secondary side of the power supply transformer. The overcurrent protective circuit temporarily interrupts the output of the switching mode power supply by preventing a drive signal from being applied to the switching transistor.
A switching mode power supply having overvoltage protection is discussed in U.S. Pat. No. 5,029,269 to Brent Elliott, et al., entitled Delayed Power Supply Overvoltage Shutdown Apparatus. This power supply uses a error detector connected to the secondary side of the power supply transformer for detecting changes in the output voltage applied to a load. The detected error controls the ON/OFF durations, i.e., duty cycle, of a switching transistor connected to the control terminal of the primary winding of the power supply transformer in order to maintain the output voltage at a fixed level. An overvoltage detector is also connected to the secondary side of the power supply transformer and is used for momentarily shutting down the power supply by blocking a drive signal from being applied to the gate of the switching transistor. If the overvoltage persists for a predetermined amount of time, the power supply is shut down permanently.
A switching mode power supply having surge voltage prevention is discussed in U.S. Pat. No. 5,099,406 to Kosuke Harada, et al. entitled DC-DC Converter With Surge Voltage Prevention. This power supply has a switching transistor connected to the control terminal of the primary winding of the power transformer. A snubber circuit is connected in parallel across the drain and source of the switching transistor to protect the switching transistor and the windings of the power transformer from surge voltages. This method of protection, however, can not completely prevent the occurrence of surge voltages.